Interpretive Summary: Over the past 25 years, the fertilizer value of dairy manure has become less appreciated as the use of inexpensive fertilizers has become widespread. For example, when calculating fertilizer application rates for field crops, many dairy farmers do not credit the nitrogen (N) they applied in the form of manure. This may be due to many factors, including a perception that manure is an undependable nutrient source. Our estimates of manure nutrient availability to crops, or the "nutrient credit" we recommend to farmers, are based on indirect methods. The stable isotope 15N has been used extensively to directly and more precisely evaluate the availability of fertilizer N to crops. The use of 15N to directly determine the availability of manure N to crops has been less studied. This is because the labeling of manure by feeding 15N forage is expensive and laborious. This paper describes two techniques that differentially enrich in 15N 3 manure N components: urine N, fecal endogenous N, and fecal undigested feed N. The choice of labeling method to use depends on the intended use of labeled manure. Manure enriched using the forage method (all 3 manure N components labeled) and high manure 15N enrichments levels should be used for long-term manure N cycling studies. Manure enriched using the urea method (urine N and endogenous fecal N labeled) and lower 15N enrichments could be used in shorter-term studies. The use of labeled manure increases researcher and farmer confidence in determining manure nitrogen availability to subsequent crops, and provides the basis for developing alternative, economically viable and environmentally sound manure management practices.

Technical Abstract:
Current estimates of dairy manure nitrogen (N) availability to crops are based on indirect measures and vary greatly. The objective of this study was to differentially label dairy manure N components with the stable isotope 15N for direct measurement of manure N cycling in soils and availability to crops. Dairy urine and fecal N components (microbial and undigested feed N) were differentially labeled by feeding either 15N-enriched forage or urea to mature dry dairy cows (Bos taurus). 15N-enriched ammonium sulfate was used to label alfalfa (Medicago sativa L.) hay and corn (Zea mays L.) silage. These 15N-enriched forages or either single or multiple doses of 15N-enriched urea were fed for 2 to 3 d, and feces and urine were collected separately for 8 d after the initiation of 15N feeding. For both labeling techniques, 15N appeared first in urine followed by fecal microbial and undigested feed N. For the forage method, the proportionate combination of feces excreted before and after peak 15N excretion levels would achieve uniform labeling of fecal N components. For the urea method, no undigested feed N in feces was labeled since 15N-enriched forage N was not fed. The choice of which labeling method to use depends on the intended use of labeled manure. Manure enriched using the forage method and high manure 15N enrichments levels should be used for long-term manure N cycling studies. Manure enriched using the urea method and lower 15N enrichments could be used in shorter-term studies.